feat(anim): animation ergonomics — named clips, Anim.play/Motion.to, frame events, fluent Tween handles (#48)

The ergonomic layer over the engine-owned SpriteAnim/Motion systems (#43):

- Named clips: Anim.clip("run", frames, fps, mode) registers a clip by name and
  Anim.play(entity, "run") plays it; Anim.play(entity, fps, frames, mode) sets
  the clip directly. A name-keyed registry in systems.ludic.
- Frame events: Anim.on_frame(entity, frame) arms optional SpriteAnim
  event_frame/event_fired fields; the engine flags the tick the clip first lands
  on that frame, and Anim.fired(entity) reads it — the game reacts, so it stays
  inside the no-runtime-dispatch event model.
- Motion.to(entity, from, to, dur, ease) starts a value tween over the Motion
  component in one call (reflection-ABI writes, resetting the timer).
- Fluent Tween handles (runtime/native/tween.ludic): Tween.to / Tween.chain /
  Tween.delay build a sequenced, disposable handle advanced by a new engine-owned
  system (esys_tween, run each Update tick); Tween.value / Tween.done /
  Tween.parallel / Tween.stop read and control it. The 1-arg Tween.done(handle)
  is disambiguated from the 2-arg pure Tween.done(timer, dur).

Splicing: g_uses_anim_rt pulls in systems.ludic; g_uses_tween_rt pulls in
tween.ludic and inserts esys_tween into the Update phase. All integer and
deterministic, so animation and motion reproduce exactly under replay/lockstep.

Worked example + regression: examples/library/anim_sugar.ludic
(4 8 2 1 0 100 100 0 0 1 20 20 30 0 1). Twelve new docs pages (Anim, the new
Motion namespace, Tween handles). Full suite 77 passed, self-host C-free fixpoint
intact, no golden drift.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

changes/anim-ergonomics.md

@@ -0,0 +1,12 @@
+bump: minor
+type: feat
+**Animation ergonomics (#48).** An ergonomic layer over the engine-owned
+SpriteAnim/Motion systems: `Anim.clip` registers named spritesheet clips and
+`Anim.play(entity, "run")` plays one (or `Anim.play(entity, fps, frames, mode)`
+directly); `Anim.on_frame` / `Anim.fired` arm and read frame events (the engine
+flags the tick a clip lands on a frame, gameplay reacts); `Motion.to` starts a
+value tween over the Motion component in one call. New fluent, engine-advanced
+`Tween` handles — `Tween.to` / `Tween.chain` / `Tween.delay` build a sequence,
+`Tween.value` / `Tween.done` / `Tween.parallel` / `Tween.stop` read and control
+it — advanced each Update tick by an engine-owned system. All integer and
+deterministic, so animation and motion reproduce exactly under replay/lockstep.